This invention relates to an echo cancellation system, and more particularly, to an echo cancellation system for use in pulse echo ultrasonic inspection systems.
A significant limitation of pulse echo ultrasonic inspection systems is the inability to detect flaws or foreign objects near the surface of a part.
In pulse echo inspection systems, an echo is received from the front surface of the test item. This is due to the acoustic impedance mismatch at the delay line/part interface. The front surface echo effectively masks any echoes from flaws or foreign objects very close to the surface. The presence of a piece of backing paper between the first and second plies in a graphite laminate is virtually undetectable. The usual method for overcoming this problem is to use a higher frequency transducer. This provides better near surface resolution by reducing the time duration of the interface echo. There are two main drawbacks to this method. First, since ultrasonic attenuation increases with frequency, echoes from defects deep in a part will be too weak for detection. Secondly, higher frequency transducers are more sensitive to porosity and other fine structure properties which might overshadow more important flaws like delaminations. Another method of detecting near surface flaws is to use TTU (through-transmission technique). Unfortunately, TTU is not sensitive enough to detect some types of foreign materials and it does not provide any depth information.
Prior patent literature discloses signal canceling circuits as seen for example in U.S. Pat. No. 4,093,923. In U.S. Pat. No. 4,093,923 a reference signal is stored in memory and subtracted from a real signal. In U.S. Pat. No. 4,093,923 the signal cancelling circuit is used to cancel constant, low frequency (60 Hz) background noise, whereas the present echo cancellation system is utilized to eliminate a complex high frequency signal. The signal cancelling circuit of U.S. Pat. No. 4,093,923 requires a constant phase signal in contrast to the present system where the signal is independent of phase. In the present echo cancellation system, the reference signal is clocked out of memory when the front surface echo is detected so any changes in the transducer part surface distance does not effect the echo cancellation. A further distinct difference from the prior art is that the present echo cancellation device uses a high speed analog to digital converter and digital memory whereas the prior art signal cancelling circuit of U.S. Pat. No. 4,093,923 uses sample and hold circuitry to acquire and store the reference waveform. The sample and hold circuit of the aforementioned prior art patent reference is suitable for a constant 60 Hz sine wave where about ten samples would be more than adequate, but in the case of a complex waveform requiring several hundred or more samples, the prior system would be impractical due to complexity and cost. By using an A to D converter and digital memory, the present echo cancellation system can store several thousand samples at minimal cost.